Machines such as, for example, wheel loaders, dozers, backhoes, dump trucks, and other heavy equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more ground engaging devices. Such machines usually include conventional manual or automatic transmissions having a discrete number of step-changed output ratios (gears) to control the speed and torque of the ground engaging devices. The output ratios correspond to travel speed ranges, each having a predefined maximum travel speed. For example, an operator can place a wheel loader in a second of available gears and, when the accelerator pedal is fully depressed, the wheel loader may accelerate to a maximum travel speed of about 8 mph.
Consequently, operators are accustomed to using certain gears for accomplishing specific tasks. For example, a wheel loader operator may know that the speed range of second gear is appropriate for effectively dozing a mound of earth of a given size. Likewise, the operator may know that the speed range of third gear is appropriate for safely traveling across a worksite with a full load. Additionally, the speed range associated with each gear of a given machine type (e.g., wheel loader) is fairly standard across different machines and different manufacturers. Therefore, operators have become comfortable using specific speed ranges for specific tasks, despite using a variety of machines.
Recently, however, conventional manual and automatic transmissions are being replaced by continuously variable transmissions (CVT). A CVT provides an infinitely variable torque-to-speed output ratio within its overall range. Although a CVT improves engine efficiency, it does not behave in the manner discussed above with respect to the discrete speed ranges of earlier transmission types. Specifically, fully depressing the accelerator pedal in a machine with a CVT will cause it to accelerate to a maximum possible speed, as the transmission uses its full range of output ratios, and the engine output will increase to a maximum possible speed. Therefore, operators must hold the accelerator pedal in an intermediate position in order to travel at a desired speed less than the maximum. This proves very difficult in rough terrain common to many worksites and causes undue operator fatigue. Accordingly, there is a need to limit the travel speed of a machine with a CVT in an alternative manner.
One such method of machine control is described in U.S. Pat. No. 6,247,378 (the '378 patent) by Newendorop et al., issued on Jun. 19, 2001. The '378 patent describes a control system for a machine having an electric CVT. Specifically, the control system is described as having a control lever disposed within a guide slot, and one or more speed adjustors for setting a maximum speed limit. The system allows an operator to slide the control lever to a set position corresponding to a desired speed limit, and to activate the speed adjustors to either set or change the speed limit. Accordingly, the machine travel speed is restricted between zero and the set speed limit.
Although the '378 patent may alleviate some of the aforementioned problems associated with travel speeds of a machine with a CVT, it does not offer discrete, selectable speed ranges, which correspond to the speed ranges of the earlier step-change transmissions. As discussed above, the operator may be more cognizant of which of the gears offers an appropriate speed range, rather than what specific speed is appropriate (e.g., 5 mph). In fact, an operator may be unaware of the speed appropriate for accomplishing the specific task. This lack of awareness may occur because an operator is usually unavailable to watch the speedometer during completion of a work task, and, therefore, would be unable to easily identify and correctly select a specific maximum speed.
The system of the present disclosure solves one or more of the problems set forth above.